Lead Removal By Scallop Shell Used for Crystallization Phosphorus Removal

Abstract

Previously, we reported that on immersing the composite of crashed scallop shells and gypsum into KH₂PO₄ aqueous solution, DCPD(CaHPO₄・2H₂O) crystal precipitated on the surface of the composite and grew with increasing immersion time. In this study, the feasibility of using the scallop shells with DCPD precipitate after phosphate ion removal as an adsorbent of lead in waste water was evaluated. When the scallop shells with DCPD precipitate were immersed in an aqueous solution containing 50ppm Pb²⁺ at pH3, the concentration of Pb²⁺ in the solution deceased with increasing immersion time. The Pb²⁺ removal rate largely depended on the amount of DCPD precipitate on the surface of the scallop shells and increased with the amount of DCPD precipitate. From the results of SEM observation, EDX and XRD analysis, it was found that after immersing scallop shells with DCPD precipitate in the Pb²⁺ aqueous solution, DCPD converted into HAP(Ca₁₀(PO₄)₆(OH)₂) and hydroxypyromophite(Pb₅PO₄)₃OH, HPY) whisker precipitated on the surface of HAP crystal. When DCPD converts into HAP, surplus PO₄^3- ions dissolve into the solution and react with Pb²⁺ ions to form Pb₅PO₄)₃OH. There results show that the scallop shells used for the adsorbent of phosphate ions would be reused as the adsorbent of lead in wastewater.